1. Field of the Invention
The present invention relates generally to cardiac pacing using an implantable cardiac stimulator, and more particularly to verification of capture of the atrium of the heart following application of an electrical stimulating pulse by the cardiac stimulator.
2. Background Information
An implantable cardiac stimulator, specifically a pacemaker, "captures" the heart by delivering an electrical pulse via an electrode to the myocardium of a selected chamber during an interval in the cardiac cycle in which the cardiac tissue is excitable. The electrical pulse causes depolarization of cardiac cells and a consequent contraction of the chamber, provided that the energy of the pacing pulse as delivered to the myocardium exceeds a threshold value.
It is desirable to adjust the cardiac stimulator so that the pulse energy delivered to the myocardium is at the lowest level that will reliably capture the chamber. Such a level assures therapeutic efficacy while maximizing the life of the implanted battery. Because the threshold for capture varies from one implantation to another, and can change over time, it is also desirable that the pulse energy delivered by the cardiac stimulator to the myocardium be adjustable during and subsequent to implantation. Adjustment can be effected manually from time to time through use of an external programmer that communicates with the implanted pacemaker. It would be desirable, however, to provide a cardiac stimulator that adjusts the pulse energy automatically and dynamically in response to changes in the capture threshold.
Changes in capture threshold can be detected by monitoring the efficacy of stimulating pulses at a given energy level. If capture does not occur at a particular stimulation energy level which previously was adequate to effect capture, then it can be surmised that the capture threshold has increased and that the stimulation energy level should be increased. On the other hand, if capture occurs consistently at a particular stimulation level over a relatively large number of successive stimulation cycles, it is possible that the stimulation threshold has decreased and that stimulation energy is being delivered at a level higher than necessary. This can be verified by lowering the stimulation energy level and monitoring for loss of capture at the new energy level.
For automatic and dynamic adjustment of the stimulation energy level to be successful, it is necessary for the implantable cardiac stimulator to be able to verify that capture has occurred. Verification of capture in the ventricle has been accomplished by detecting a characteristic electrical potential in the heart evoked by the stimulating pulse. If capture has not occurred, there will be no characteristic evoked potential to detect. It follows that each time a stimulating pulse is delivered to the ventricle, the heart can be monitored during an appropriate period of time thereafter to detect the presence of the evoked potential, and thereby verify capture. In practice, however, reliable detection of the evoked potential of the ventricle is not a simple matter. This is because the evoked potential is small in amplitude relative to the residual polarization charge on the electrode resulting from the stimulation pulse. The residual charge decays exponentially but tends to dominate the evoked potential for several hundreds of milliseconds thereafter. Detection of the evoked potential of the atrium is even more difficult, as the amplitude of the atrial evoked potential is quite small and difficult to discriminate from noise, other cardiac signals, and the residual polarization charge on the electrode.
It would be desirable to provide a method for analyzing cardiac signals, for use in an implantable cardiac stimulator, that permits verification of capture of the atrium in the presence of a residual charge from a preceding stimulation pulse and in the presence of other signals. This and other desirable goals are met by the present invention.